For the convenience of description, reference is now made to the attached FIG. 7. As typically shown in FIG. 7, a prior art bicycle rear derailleur a has a chain guide d which rotatably supports a guide pulley b and a tension pulley c. This chain guide d is supported, via a shift link mechanism such as a parallelogram pantograph mechanism g, on a rear end plate e at an end portion of a bicycle frame, or on a bracket f attached to the rear end plate e so as to pivot about a predetermined axis while being urged in a direction to tension a chain C.
The pantograph mechanism g includes: a link base h supported by the bracket f; inner and outer link members i, j which are pivotally connected at their respective base end portions to the link base h and extended forwardly of the bicycle; and a movable member k which is pivotally connected to each free end portions of the link members i, j. The movable member k rotatably supports the chain guide d.
The chain guide d is elastically urged by a coil spring (not shown in FIGS.) in a direction to tension the chain C, i.e. in a clockwise direction in FIG. 7. This removes slack in the chain C and gives the chain C a predetermined level of tension.
With the above-mentioned arrangement, when the control cable T, which is connected to the pantograph mechanism g, is pulled or released by a control lever (not shown), the pantograph mechanism g deforms to cause the chain guide d, which is supported by the movable member k, to move axially of a hub shaft n, forcing the chain C to move to a selected sprocket m of a sprocket cluster M.
In this type of bicycle rear derailleur, it is desirable for the sake of improved speed change performance that an edge of the guide pulley b and an edge of each sprocket m of the sprocket cluster M should be within appropriate proximity. This is because the guide pulley b is directly responsible for shifting a portion of the chain ahead of the sprocket cluster M from a sprocket presently engaged with the chain to a target sprocket, and if the edge of the guide pulley b is too far away from the edge of the sprocket cluster M, the guide pulley b will have to travel over a long distance for completing a chain shift operation, resulting in a poor derailleur response.
For this purpose, it is desirable to design a speed change assembly to function in such a way that the guide pulley b would maintain a substantially constant distance with each opposing sprocket m of the sprocket cluster M while the guide pulley moves over an entire range of its travel. In other words, the chain guide d should not move in parallel with the hub shaft n, but shoud move substantially in parallel with a line running through an edge of each sprocket m of the sprocket cluster M. Since the rear sprocket cluster M is usually configured in a manner that a sprocket of a greater diameter is placed closer to the hub shaft, it is necessary for the guide pulley b to moved away from the hub shaft n as it moves axially inward of the hub.
In a prior art bicycle derailleur a, the above-described purpose is achieved in the following means:
As shown in FIG. 7, in a prior art bicycle dereilleur a, the link base h is supported below the hub shaft n, and from this link base h, the inner and outer link members i, j are extended forwardly of the bicycle to configure the pantograph mechanism g, in which connecting pins p, q of the inner and outer link members i, j are slanted off the vertical by a predetermined angle, or otherwise, the link base h is supported pivotally about the bracket f. With this arrangement, it is possible to move the guide pulley b in a manner as shown in FIG. 6, that the guide pulley b travels rearwardly downward of the bicycle as it moves axially inward of the hub shaft, so that the distance between the edge of the guide pulley b and the edge of each sprocket m is maintained substantially constant.
There is a problem, however, with this type of prior art arrangement. As shown in FIG. 6, when the pantograph mechanism g deforms, the chain guide d supported on the movable member k approaches the vertical V which runs through the hub shaft, making a travel 1 of the guide pulley b away from a sprocket m substantially smaller than its actual travel L of the guide pulley b. As a result, it is necessary in the prior art arrangement that as the sprocket's gear ratio increase, a greater slant angle has to be provided for the connecting pins p and q, with greater amount of deformation required of the pantograph g, and greater amount of pull required of the control cable T. All of these pose a problem of increased overall size of the speed change assembly and the speed change lever.
Another problem with the prior art speed change assembly is that the great deformation of the pantograph link mechanism g and the resulting length of the pull of cable T pose a hurdle against improved operatability of the speed change operation.
Further, since the guide pulley b has to travel over a long distance, the slack in the chain C has to be correspondingly great, resulting in a long and heavy weight of the chain C, putting a limit to the weight reduction of a bicycle.
There is still another disadvantage in the prior art speed change assembly. Since the rear derailleur a is located at a laterally outermost position of a bicycle, it is very susceptible to damage in case of a bicycle rollover. Especially on bicycles specifically designed for off-road cycling, popularly known as mountain bikes, the end of the rear derailleur a is apt to catch bush or other obstacles, interfering with the speed change function.
In an attempt to solve these problems the applicant of the present invention proposed, in the Japanese Patent Application No. Hei 3-65539, a bicycle speed change assembly comprising: a pantogtaph link mechanism including a link base supported on a chain stay ahead of a sprocket cluster mounted on a hub shaft, inner and outer links each having a base end pivoted to the link base and extending rearward, and a movable member pivoted to respective free ends of these inner and outer links; a chain guide rotatably supporting a guide pulley and a tension pulley, supported on the movable member of the pantograph mechanism to pivot about a shaft in parallel to the hub shaft while being elastically urged to tension a chain; and a guide pulley travel control means for moving the guide pulley inward axially of the hub shaft and forwardly downward substantially along a radial path of the sprocket cluster when the pantograph mechanism is deformed.
In this particular bicycle speed change assembly, it is possible to move the guide pulley along a path substantially in parallel with a radius of the sprocket cluster, or in other words, the guide pulley will travel substantially in parallel with a generatrix of an imaginative frustum defined by connecting edges of the sprockets on the sprocket cluster. Furthermore, since the guide pulley moves along a generatrix which runs in a forwardly downward direction, the chain's engaging angle with each sprocket is substantially constant.
This means that according to this prior art invention, the travel distance of the guide pulley necessary to move the chain from the smallest sprocket to the largest sprocket is minimized.
Thus, the pivotal movement required of the inner and outer link mambers is smaller than before, making it possible to reduce the size of the speed change assembly. The amount of pull needed for the control cable is also smaller, resulting in remarkably improved efficiency and operatability in the speed change operation, making it possible to reduce the size of the control lever assembly.
Furthermore, it requires a smaller travel distance of the speed change lever, and therefore, it is also possible to miniaturize the speed change lever assembly.
Another advantage is since the speed change assembly is mounted below a chain stay, the speed change assembly is not an outermost component of the bicycle width. This provides less chance for major damage to the speed change assembly in case of a bicycle rollover.
This prior art application discloses, in claim 2 thereof, a guide pulley control means wherein connecting pins for the inner and outer link members of the pantograph mechanism are slanted off the vertical.
When the connecting pins for the inner and outer link members are slanted, it is possible to cause the movable member of the pantograph machanism to move inward axially of the hub shaft and forward longitudinally of the bicycle, and in addition, downward vertically of the bicycle. As a result, it is possible to move the guide pulley inward axially of the hub shaft, and at the same time, downward forwardly along a radial path of the sprocket cluster. In other words, the guide pulley will move substantially along a forwardly downslope generatrix of the frustum defined by connecting the edges of each sprocket on the sprocket cluster. The speed shift performance is thus remarkably improved.
However, when the connecting pins are slanted in the above-mentioned bicycle speed change assembly to build a speed change assembly known a slant pantograph type, there is a problem that an inner side of the inner link member or that of the movable member can interfere with rear wheel spokes.
This is because the speed change assembly is mounted below the bicycle frame ahead of the hub shaft, and hance the pantograph mechanism is located inwardly of the bicycle width. If the connecting pins are slanted in this arrangement, the inner side of the inner link member or that of the movable member is apt to interfere with rear wheel spoke rotational plane. This puts limit to the extent of pantograph deformation, and to a pivoting range of the movable member as it moves inward of the bicycle width. As a result, the moving range of the movable member must be shifted outwardly of the bicycle width.
However, if the moving range of the movable member is shifted outwardly of the bicycle width, the movable member or the outer link member will stick out of the bicycle width when the movable member is moved to a position where the chain engages the smallest sprocket. This is not only disadvantageous in terms of bicycle apperarance but recurrence to the conventional problem that the speed shift assembly is susceptible to damage in case of bicycle rollover.
Therefore, it is a purpose of the present invention to provide a bicycle speed change assembly supported on a chain stay ahead of a sprocket cluster wherein a guide pulley moving area is secured without increasing protrusion of the speed change assembly out of the bicycle width, without increasing risk for damage to the speed change assembly in case of bicycle rollover, and without decreasing appearance of a bicycle.
The present invention provides a bicycle speed change assembly comprising: a pantogtaph link mechanism including a link base supported on a chain stay ahead of a sprocket cluster mounted on a hub shaft, inner and outer link members each extending rearward, and having a base end pivoted by a connecting pin to the link base, and a movable member pivoted by connecting pins to respective free ends of the inner and outer link members; and a chain guide rotatably supporting a guide pulley and a tension pulley, the chain guide being supported on the movable member of the pantograph mechanism to pivot about an axis in parallel with the hub shaft while being elastically urged to tension a chain. The present invention is characterised in the following means:
Each of the connecting pins is slanted off the vertical for moving the guide pulley inward axially of the hub shaft and forwardly downwardly of the sprocket cluster when the pantograph mechanism is deformed, and each of the inner and outer link members have a top surface, a plane containing at least a portion of the top surface of the inner link member adjacent to the movable member is displaced downward axially of the connecting pins from another ploane containing a portion of the top surface of the outer link member adjacent to the movable member.
Other purposes, characteristics and advantages of the present invention will be understood clearly through the description of a preferred embodiment to be described hereinafter by referring to the attached figures.